An MOSFET (Metal Oxide Semiconductor Field Effect Transistor), an IGBT (Insulated Gate Bipolar Transistor) or the like, for example, has been used as a power semiconductor device. With higher voltage and larger current, higher speed of switching has often been demanded on the power semiconductor device.
A parasitic diode is built in the MOSFET. When an n-channel type MOSFET is used, for example, as a DC-DC converter having an inductance load, an inverter or the like, the parasitic diode is used for a turning current by applying a current to forward direction.
Technology controlling a carrier life time in an n-type drift layer to be shorter by irradiating charged particles such as electron, H+, He2+ or the like has been known for improvement reverse recovery characteristics of the parasitic diode.
Electrons and holes carried into the n-type drift layer in the n-channel type MOSFET is caused by a bipolar action due to a forward current in the parasitic diode. When crystalline defects as recombination centers by irradiating charged particles such as electron or the like, a recombination lifetime is shortened to improve the reverse recovery characteristics.
As a gate electrode of the IGBT is applied to a minas voltage compared to an emitter electrode in a case of turn-off, electrons carried from an n+-type emitter layer are stopped. On the other hand, electrons are leaved in an n-type drift layer, which is called an n-type base layer, and a large portion of holes is carried out to the emitter electrode. However, a part of the holes are leaved in the n-type drift layer. As the residual electrons and holes are diminished by the recombination, the carrier lifetime can be controlled to be shorter by irradiating H+, He2+ or the like other than electron. Hereafter, proton, deuteron, helium ion or the like which is an ion with atomic number being one or two, is described as a light ion.
A relationship between the recombination center and the crystalline defect is mentioned below. In a semiconductor material having a large band gap such as silicon, a recombination is caused through the recombination center. Generating a recombination center near a center of the band gap is an ideal method for effectively driving the recombination process between electrons and holes. It has been well-known that the crystalline defects are intentionally induced by irradiating the light ion or the like so as to introduce the recombination centers in near center of the band gap.
A method mentioned below is disclosed in Japanese Patent No. 2963204, for example, as forming the crystalline defects. A metal absorber, for example, aluminum or the like is configured on a surface of a collector electrode in a back surface of an IGBT and a mask, for example, a stainless-steel or the like having an opening is configured in a surface of the IGBT. A light ion, for example, helium or the like is irradiated through the mask. A light ion particles through a fine opening in the mask are set in a position of a projected rage in n−-base layer by determining acceleration energy and a thickness of the absorber.
The characteristics of the IGBT disclosed in Japanese Patent No. 2963204 are irradiated with the light ions by using the mask are improved as compared to a sample irradiated without a mask. However, as the absorber is 30 μm and the mask is 50 μm thick to the acceleration energy being 20 MeV, a radius of the mask opening is 140 μm or the like.
Problems with difficulty are generated, for example, further miniaturization of the opening and increasing position accuracy in plane. In other word, the conventional method includes a difficult problem, for example, a selective irradiation cannot be performed. In the conventional method, the light ions can be irradiated both in a region being necessary with controlling the carrier life time and in another region being unnecessary for retaining breakdown voltage.